Cellular telephone location system

ABSTRACT

A cellular telephone location system for automatically recording the location of one or more mobile cellular telephones comprises three or more cell site systems 12. Each cell site system is located at a cell site of a cellular telephone system. Each cell site system includes an antenna that may be mounted on the same tower or building as the antenna employed by the cellular telephone system and equipment that may be housed in the equipment enclosure of the corresponding cell site. The cell site systems are coupled via T1 communication links 14 to a central site 16. The central site may be collocated with the cellular telephone system&#39;s MTSO. The central site 16 is further coupled to a database 20, which may be remotely located from the central site and made available to subscribers.

FIELD OF THE INVENTION

The present invention relates generally to the field of mobile cellulartelephone systems (including both analog and digital cellular systems)and more particularly relates to a system for automatically locatingmobile cellular telephones operating within a prescribed geographicarea.

BACKGROUND OF THE INVENTION

Prior to the invention disclosed herein, there has been no known systemfor automatically tracking mobile cellular telephones. Although relatedtechnologies (radio navigation systems such as direction finding andLORAN, emergency location devices for aircraft, satellite tracking andsurveillance, and the like) have been extant for many years, none ofthese technologies has been applied to automatically locate cellulartelephones as described herein. Accordingly, the background informationmost pertinent to gaining an understanding of the present inventionrelates to a cellular telephone system itself, as opposed to theperipherally related radio navigation and location technologies. Thefollowing discussion refers to FIGS. 1A-1C in providing an overview of acellular telephone technology. In addition, it should be noted that theinventive concepts disclosed herein are applicable to both analog anddigital (for example, TDMA) cellular systems that employ analog controlchannels.

Cellular telephone systems typically include many cell sites and acentrally-located cellular switch, called a Mobile Telephone SwitchingOffice (MTSO). There are typically sixty to one hundred cell sites inlarge cities and fifteen to thirty cell sites in smaller cities. Cellsites are usually spaced at distances of one-half to twenty miles. Eachcell site generally comprises one or more antennas mounted on atriangular platform. The platform is placed on a tower or atop a tallbuilding, preferably fifty to three hundred feet above the surroundingterrain.

The fundamental idea behind a cellular system is frequency reuse. Thisconcept of frequency reuse is implemented by employing a pattern ofoverlapping cells, with each cell conceptually viewed as a hexagon. Thisconcept is illustrated in FIG. 1A, which depicts a layout for a cellularsystem employing seven distinct sets of frequencies. In this figure,each shading pattern represents a unique frequency set. FIG. 1Cschematically depicts the main components and arrangement of cellulartelephone system. As discussed above, frequency reuse allows thecellular system to employ a limited number of radio channels to servemany users. For example, FIG. 1A depicts an area served by 14 cells,divided into two clusters. Each cluster contains seven cells. A separateset of channels is assigned to each cell in a cluster. However, the setsused in one cluster are reassigned in the other cluster, thus reusingthe available spectrum. The signals radiated from a cell in channelsassigned to that cell are powerful enough to provide a usable signal toa mobile cellular telephone within that cell, but preferably notpowerful enough to interfere with co-channel signals in distant cells.All cellular telephones can tune to any of the channels.

The Federal Communications Commission (FCC) has allocated a 25 MHzspectrum for use by cellular systems. This spectrum is divided into two12.5 MHz bands, one of which is available to wire line common carriersonly and the other of which is available to non-wire line commoncarriers only. In any given system, the non-wire line service provideroperates within the "A side" of the spectrum and the wire line provideroperates within the "B side" of the spectrum Cellular channels are 30KHz wide and include control channels and voice channels. In particular,the twenty-one control channels for "A" systems are numbered 313 through333 and occupy a 30 KHz band of frequencies 834.390 MHz to 834.990 MHz.The control channels for "B" systems are numbered 334 through 354 andoccupy 835.020 MHz to 835.620 MHz. Each cell site (or, where a cell siteis "sectored" as described below, each sector of that cell site) usesonly a single control channel. The control channel from a cell site to amobile unit is called the "forward" control channel and the controlchannel from the cellular telephone to the cell site is called the"reverse" control channel. Signals are continuously broadcast over aforward control channel by each cell site. In contrast, signals arediscontinuously (periodically) broadcast by the cellular telephones overa reverse control channel. If the cell sites are so close to one anotherthat control channels using the same frequency interfere with eachother, the control channel at each cell site is further qualified by adigital color code ranging from zero to three. This allows each cellsite to be uniquely identified, for example, within a range of twenty tothirty miles.

Directional cell site antennas may be used to reduce co-channel andadjacent-channel interference. FIG. 1B illustrates how sectored antennasmay be used to reduce such interference. The circles represent cellsites and the broken lines represent the azimuthal edges of the frontlobes of 120° directional antennas. The labels "A" , "B" , and "C" referto channel sets, cells, and cell sites simultaneously. The labels "1" ,"2" , and "3" refer to directional antennas and sectors of cellssimultaneously. Thus, for example, if a particular channel is assignedto sector 1 of cell B and adjacent channels are assigned to cells A andC, these adjacent channels should be assigned to sector 1 in cells A andC.

When a cellular telephone is first turned on, it scans all forwardcontrol channels, listening for the channel with the strongest signal.The telephone then selects the forward control channel with thestrongest signal and listens for system overhead messages that arebroadcast periodically, for example, every 0.8 seconds. These overheadmessages contain information regarding the access parameters to thecellular system. One such access parameter is the frequency ofregistration, which refers to how often a given telephone must informthe system that the telephone is within the system's geographicconfines. Registration frequencies typically range from once per minuteto once per thirty minutes.

The overhead messages also contain busy/idle bits that provideinformation about the current availability of the reverse controlchannel for that cell. When the reverse control channel becomes free, asindicated by the busy/idle bit, the cellular telephone attempts toregister itself by seizing the reverse control channel. Cellulartelephones re-register themselves at the rate determined by the cellularsystem. Registration parameter requirements are determined by eachcellular system. For example, the options include (1) 7-digit NXX-XXXX,(2) 3-digit NPA, and (3) 32-bit electronic serial number. Each of theseoptions constitutes a digital word. Because of sync bits and errorcorrection techniques, each digital word is 240 bits long. With aninitial 48-bit sync stream, each cellular telephone transmission is aminimum of 288 bits long, and as long as 1488 bits. Moreover, eachdiscontinuous transmission by a cellular telephone includes a period ofunmodulated carrier. Therefore, an average transmission on the reversecontrol channel lasts about 100 milliseconds. Cellular telephones alsotransmit in response to pages by the cellular system, as well as inresponse to user-initiated calls. The term "paging" is used to describethe process of determining a mobile telephone's availability to receivean incoming call. The complementary function of initiating a call by themobile telephone is called "access." The paging and access functionsoccur on the control channels.

When turned on but not in active use, a mobile cellular telephoneperiodically scans the control channels assigned to the system and marksfor use the strongest carrier found. With the mobile receiver tuned tothis strongest carrier, the cellular telephone continuously decodes adigital modulating data stream, looking for incoming calls. Any call toa mobile terminal is initiated like a normal telephone call. A seven- orten-digit number is dialed and the telephone network routes the call toa central computer. The number is broadcast on the control channels ofevery cell in the system. When a called telephone detects its number inthe incoming data stream, it sends its identification back to thesystem. The system uses a digital message on the control channel todesignate a channel for the telephone to use. The telephone tunes tothis channel and the user is then alerted to the incoming call. Asimilar sequence is involved when a cellular telephone user originates acall. The user dials the desired telephone number into a register in thetelephone. This number is transmitted over the control channel to thenearest cell (i.e., the cell with the strongest carrier). The systemcomputer then designates a channel for the call and the mobile unit isautomatically tuned to that channel.

The cellular telephone industry has enjoyed widespread success in itsrelatively brief lifetime. New subscribers, apparently recognizing themany advantages in being able to initiate and receive calls while awayfrom home, are being enrolled in ever-increasing numbers. Indeed, inmany cities, the competition between the A and B sides to enlist newsubscribers is fierce. Accordingly, there is a great need for newservices to offer current and potential subscribers. The presentinvention sprang from the recognition that mobility, the main advantageoffered by a cellular system, is also a disadvantage in certainsituations. For example, a lost or stolen cellular telephone isdifficult to recover. Thus, a system that could automatically locate thetelephone would be quite beneficial to users. In addition, if thecellular telephone were in an automobile and the automobile were stolen,a system that could locate the telephone would also be able to locatethe automobile, thus providing a valuable service to users. Moreover,there are situations where the user of a cellular telephone may becomelost. An example of such a situation is where the user is driving in anunknown area at night with his telephone in the car. Again, it would bea great advantage for the system to be able to automatically locate thetelephone and, upon request, inform the user of his location. Similarly,a cellular telephone user experiencing a medical emergency who dials anemergency telephone number (for example, 911) may not be able to tellthe dispatcher his location. Prior art systems are unable to trace acall from a cellular telephone. Therefore, a cellular telephone user insuch a situation would be in a dire predicament. Once again, it would behighly advantageous for the system to be able to ascertain the user'slocation and provide this information to emergency medical personnel.There would be numerous other applications for a system that couldautomatically locate a cellular telephone.

SUMMARY OF THE INVENTION

The present invention provides a cellular telephone location system fordetermining the locations of multiple mobile cellular telephones eachinitiating periodic signal transmissions over one of a prescribed set ofcontrol channels. The invention may be embodied in a system that employsmuch of the existing infrastructure of a cellular system. For example,as described below in greater detail, a cellular telephone locationsystem in accordance with the present invention may employ the cellularsystem's towers and cell site enclosures. In this sense, the cellulartelephone location system may be overlaid on the cellular system.

There are numerous advantages provided by monitoring control channels totrack the locations of cellular telephones. First, a voice channel is anexpensive and relatively scarce resource. Cellular systems typicallyrequire approximately six to eight seconds to allocate a voice channelto a specific telephone. If voice channels were employed for locationtracking, the cellular telephone would have to be called and commandedto initiate a voice channel call every time a location sample were to betaken. This would be both expensive and time consuming. Thus, it wouldbe extremely inefficient for a location system to require the telephoneto initiate periodic voice channel transmissions. Second, each voicechannel transmission adds a call record in an associated billing system.Therefore, a large burden would be placed on the billing system if thelocation system were to require periodic voice channel transmissions. Incontrast, control channel transmissions already occur periodically incellular systems. Thus, the present invention is compatible withexisting cellular telephone protocols and would not require the cellularsystem or the individual cellular telephones to be modified. Third,since the frequency of control channel transmissions is softwarecontrollable, a location system in accordance with the present inventioncould control the frequency of control channel transmissions and offerdifferent subscribers different location information update rates.Fourth, another advantage afforded by monitoring control channeltransmissions is in connection with energy efficiency. Control channeltransmissions are very short and require little power in comparison tovoice channel transmissions. Accordingly, requiring periodic voicechannel transmissions would cause a significant battery drain in theindividual cellular telephones. This is avoided by monitoring controlchannels.

Accordingly, there are significant advantages afforded by monitoringperiodic control channel transmissions to automatically locate mobilecellular telephones. However, monitoring control channels requiresdetection of such weak, short duration signals that have travelled largedistances (for example, twenty-five miles). The present inventors havedeveloped highly sophisticated signal processing methods and apparatusto detect extremely brief, low power control channel signals. Both theconcept of monitoring periodic control channel transmissions, as opposedto voice channel transmissions, and the particular way in which thisfunction is carried out represent significant technologicaladvancements.

An exemplary embodiment of the present invention comprises at leastthree cell site systems and a central site system. Each cell site systemcomprises an elevated ground-based antenna; a baseband convertor forreceiving cellular telephone signals transmitted by the cellulartelephones and providing baseband signals derived from the cellulartelephone signals; a timing signal receiver for receiving a timingsignal common to all cell sites; and a sampling subsystem for samplingthe baseband signal and formatting the sampled signal into frames ofdigital data. Each frame includes a prescribed number of data bits andtime stamp bits, wherein the time stamp bits represent the time at whichthe cellular telephone signals were received. The central site systemcomprises means for processing the frames of data from the cell sitesystems to generate a table identifying individual cellular telephonesignals and the differences in times of arrival of the cellulartelephone signals among the cell site systems; and means fordetermining, on the basis of the times of arrival, the locations of thecellular telephones responsible for the cellular telephone signals.

In one preferred embodiment of the invention, the central site systemcomprises a correlator for cross-correlating the data bits of each framefrom one cell site with the corresponding data bits of each other cellsite. In addition, this preferred embodiment comprises a database forstoring location data identifying the cellular telephones and theirrespective locations, and means for providing access to the database tosubscribers at remote locations. The system also comprises means forproviding location data to a specific cellular telephone user uponrequest by using, for example, CPDP without setting up a voice call("CPDP" represents the Cellular Packet Data Protocol, which involvessending data over voice channels when the voice channels would nototherwise be in use). The latter feature is especially useful inconnection with laptop or handheld computers having cellular modems andmapping software.

Embodiments of the invention may also advantageously include means formerging the location data with billing data for the cellular telephonesand generating modified billing data. In this embodiment, the billingdata indicates the cost for each telephone call made by the cellulartelephones within a certain time period, the cost being based upon oneor more predetermined billing rates, and the modified billing data isbased upon a different rate for calls made from one or more prescribedlocations. For example, the system may apply a lower billing rate fortelephone calls made from a user's home or office or other geographiclocale.

Embodiments of the invention may also advantageously include means fortransmitting a signal to a selected cellular telephone to cause theselected telephone to transmit a signal over a control channel. Suchcapability would allow the system to immediately locate that telephonewithout waiting for one of its periodic control channel transmissions.

In addition, embodiments of the invention may comprise means forautomatically sending location information to a prescribed receivingstation in response to receiving a distress signal from a cellulartelephone. With this capability, emergency assistance may be provided toa user in distress. For example, when a user dials "911" the systemwould automatically tell an emergency dispatcher the user's location.

Another element of a preferred embodiment is a means for comparing thecurrent location of a given telephone with a prescribed range oflocations and indicating an alarm condition when the current location isnot within the prescribed range. Such an element could be used, forexample, to notify a parent when the child, who borrowed the parent'scar and cellular telephone to "go to the mall," has in fact gonesomewhere else. Of course, many other applications of such an alarmfunction are possible.

Yet another element of a preferred embodiment is a means for detecting alack of signal transmissions by a given telephone and in responsethereto automatically paging the given telephone to cause it to initiatea signal transmission. This would allow the system to locate a telephonethat has failed to register itself with the cellular system. Such alack-of-signal-transmission detection feature could be used, forexample, to generate an alarm for subscribers at remote locations.

In addition, preferred embodiments may also include means for estimatinga time of arrival of a given telephone at a prespecified location. Thiswould be useful, for example, in connection with a public transportationsystem to provide quasi-continuous estimated times of arrival of bussesalong established routes. Of course, many other applications of thisfeature are also possible.

Embodiments of the present invention may also comprise means forcontinuously tracking a given telephone by receiving voice signalstransmitted by the given telephone over a voice channel and determiningthe location of the given telephone on the basis of the voice signals.This voice channel tracking could be used as an adjunct to controlchannel tracking. This feature would require the location system totrack the channel assignment of each telephone whose location is to bedetermined. The tracking of channel assignments by the location systemcould employ the dynamic channel assignment protocol employed by thecellular system.

The present invention also provides methods for determining the locationof one or more mobile cellular telephones. Such methods comprise thesteps of: (a) receiving the signals at at least threegeographically-separated cell sites; (b) processing the signals at eachcell site to produce frames of data, each frame comprising a prescribednumber of data bits and time stamp bits, the time stamp bitsrepresenting the time at which the frames were produced at each cellsite; (c) processing the frames of data to identify individual cellulartelephone signals and the differences in times of arrival of thecellular telephone signals among the cell sites; and (d) determining, onthe basis of the times of arrival, the locations of the cellulartelephones responsible for the cellular telephone signals.

One preferred embodiment of the inventive method comprises estimatingthe location of a cellular telephone by performing the following steps:(1) creating a grid of theoretical points covering a prescribedgeographic area, the theoretical points being spaced at prescribedincrements of latitude and longitude; (2) calculating theoretical valuesof time delay for a plurality of pairs of cell sites; (3) calculating aleast squares difference (LSD) value based on the theoretical timedelays and measured time delays for a plurality of pairs of cell sites;(4) searching the entire grid of theoretical points and determining thebest theoretical latitude and longitude for which the value of LSD isminimized; and (5) starting at the best theoretical latitude andlongitude, performing another linearized-weighted-least-squaresiteration to resolve the actual latitude and longitude to within aprescribed number of degrees or fraction of a degree. Preferably, thecalculating step (2) comprises accounting for any known site biasescaused by mechanical, electrical, or environmental factors, the sitebiases determined by periodically calculating the positions of referencecellular transmitters at known locations.

In addition, the least squares difference is preferably given by:

    LSD=[Q.sub.12 (Delay.sub.-- T.sub.12 -Delay.sub.-- O.sub.12).sup.2 +Q.sub.13 (Delay.sub.-- T.sub.13 -Delay .sub.-- O.sub.13).sup.2 +...Q.sub.xy (Delay.sub.-- T.sub.xy -Delaye.sub.-- O.sub.xy).sup.2 ]

where, Delay₋₋ T_(xy) represents the theoretical delay between cellsites x and y, x and y being indices representative of cell sites;Delaye₋₋ O_(xy) represents the observed delay between cell sites x andy; Q_(xy) represents a quality factor for the delay measurement betweencell sites x and y, the quality factor being an estimated measure of thedegree to which multipath or other anomalies may have affected aparticular delay measurement.

Further, the inventive method may advantageously include detecting afirst leading edge of a cellular telephone signal and rejectingsubsequent leading edges of the cellular telephone signal. This allowsthe system to reduce the effects of multipath.

In addition, preferred embodiments include estimating the velocity(speed and direction) of a cellular telephone by performing stepssimilar to those performed for location estimation, including: (1)creating a grid of theoretical points covering a prescribed range ofvelocities, the theoretical points being spaced at prescribedincrements; (2) calculating theoretical values of frequency differencefor a plurality of pairs of cell sites; (3) calculating a least squaresdifference (LSD) value based on the theoretical frequency differencesand measured frequency differences for a plurality of pairs of cellsites; (4) searching the entire grid of theoretical points anddetermining the best theoretical velocity for which the value of LSD isminimized; and (5) starting at the best theoretical velocity, performinganother linearized-weighted-least-squares iteration to resolve theactual velocity to within a prescribed tolerance.

Other features of the present invention are described below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a depiction of an exemplary frequency reuse pattern employedin a cellular telephone system.

FIG. 1B is a schematic depiction of an exemplary channel assignmentpattern where cell sectoring is employed.

FIG. 1C is a schematic depiction of the basic components of a cellulartelephone system.

FIG. 2 is a schematic diagram of a cellular telephone location system inaccordance with the present invention.

FIG. 3 is a block diagram of one preferred embodiment of a cell sitesystem 12.

FIG. 4 is a block diagram of one preferred embodiment of a basebandconverter 12-3.

FIG. 5 is a schematic diagram of the data format provided by a formatblock 12-5.

FIG. 6 is a block diagram of one preferred embodiment of a central sitesystem 16.

FIG. 6A is a block diagram of a correlator for use in the central sitesystem 16.

FIG. 7 is a simplified flowchart of a preferred operating sequence ofthe central site system.

FIG. 7A is a block diagram depicting exemplary embodiments of cell sitesystems employed in a location system which performs cross-correlationsat the cell sites.

FIGS. 8A-8E are a flowchart of the operation of the cell site system 16in obtaining correlation data, time delay and frequency difference(TDOA, FDOA) data, and calculating the location of a cellular telephoneon the basis of such data.

FIG. 9 is a schematic diagram of a process for generating a modifiedbilling tape in accordance with the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Overview

Preferred embodiments of the present invention comprise a network ofreceivers located at multiple cell sites in a cellular system. Thesereceivers listen to the mobile control channel commands/responsesnormally occurring in the cellular system and estimate the physicallocation of each cellular telephone operating within the system. Basedupon the known identity of each telephone, obtained from listening tothe control channel, and the estimated physical location of thetelephone, the system provides a continuous, real time data stream to adatabase. The database may be collocated with the cellular switch or maybe in some other convenient location. The data stream provided to thedatabase comprises a set of numbers, the first number being thetelephone number of the telephone, the second number being the estimatedlatitude, longitude, and altitude of the transmitter, and the thirdnumber being the time stamp of the measurement. The database softwarethat processes the data stream may be maintained by the operator of thelocation system rather than the operator of the cellular telephonesystem, if the two are not the same.

The location system operates by using the frequencies assigned to thecontrol channels of the cellular system. Cellular telephones use thesecontrol channels to maintain regular contact with the cellular system,with the time between each contact being typically no more than thirtyminutes and generally about ten minutes. Each control channel comprisesa 10 kbps Manchester encoded data stream. There is only one controlchannel used per cellular sector or omni cell site. The location systemis capable of functioning by listening only to the control channelbroadcasts of the cellular telephones; it does not depend on controlchannel broadcasts from the cell sites. The location system preferablycomprises equipment that is located atop cellular towers (although theequipment may be located on other tall structures), in the equipmentenclosure at cells sites, and at the central switch site(s).

Referring now to FIG. 2, a cellular telephone location system inaccordance with the present invention comprises at least three, andpreferably more, cell site systems 12a, 12b, 12c, 12d. (It should benoted that this figure, as well as the other figures, is simplified inthat some elements and interconnections have been omitted. However, theinstant specification and attached drawings are sufficient to enable oneskilled in the art to make and use the invention disclosed herein.) Eachcell site system may be located at a cell site of the cellular telephonesystem; however, this is not required since additional antenna andreceiving equipment could be deployed at locations not well covered bycell sites. FIG. 2 also shows a user with a cellular telephone 10a. Asdescribed below, each cell site system includes an antenna that may bemounted on the same tower or building as the antenna employed by thecellular telephone system. In addition, each cell site system includesequipment (described below) that may be housed in the equipmentenclosure of the corresponding cell site. In this manner, the cellulartelephone location system may be overlaid on the cellular telephonesystem and thus may be implemented inexpensively. The cell site systems12a, 12b, 12c, 12d are coupled via communication links 14a, 14b, 14c,14d (for example, T1 communication links) to a central site 16. Thecentral site 16 may be collocated with the cellular telephone system'sMTSO. The central site 16 may include a disk storage device 18.

The central site 16 is further coupled to a database 20, which may beremotely located from the central site and made available tosubscribers. For example, FIG. 2 depicts a first terminal 22 coupled viaa modem (not shown) and telephone line to the database 20; a secondterminal 24 in radio communication with the database 20; and a third,handheld terminal 26, which is carried by a user who also has a cellulartelephone 10b, in radio communication with the database. The user withthe cellular telephone 10b and handheld terminal 26 may determine hisown location by accessing the database. The handheld terminal 26 mayinclude special mapping software for displaying the user's location, forexample, on a map, on the terminal 26. Moreover, the cellular telephoneand handheld terminal could be combined into one unit.

Cell Site Systems

FIG. 3 is a block diagram of one presently preferred embodiment of acell site system 12. Before discussing the exemplary cell site systemdepicted in this figure, it should be noted that there are twoalternative preferred embodiments for the equipment at each cell site,with the particular embodiment for a particular cellular systemdependent upon desired cost.

The first embodiment is the most preferred embodiment, and comprises (1)an antenna suited for receiving signals in the cellular frequency band;(2) a low delay bandpass filter with a bandwidth of 630 KHz locatedwithin ten to fifteen feet of the cellular antenna for removing adjacentchannel interference; (3) an amplifier of sufficient gain to compensatefor cable loss in the distance from the amplifier to the next filter,which is typically the height of the antenna tower plus any horizontaldistance over which the cable is routed; (4) a set of twenty-oneindividual low delay bandpass filters, each with a bandwidth of 30 KHzcentered about one of the twenty-one control channels; and (5) a set oftwenty-one automatic gain control circuits with a dynamic range of 70 dB(note that not all of these components are depicted in FIG. 3). Thisembodiment is preferred because of its superior interferencediscrimination and rejection.

The second embodiment comprises (1) an antenna suited for receivingsignals in the cellular frequency band; (2) a low delay bandpass filterof bandwidth 630 KHz located within ten to fifteen feet of the cellularantenna for removing adjacent channel interference; (3) an amplifier ofsufficient gain to compensate for cable loss in the distance from theamplifier to the next filter, which is typically the height of theantenna tower plus any horizontal distances over which the cable isrouted; (4) a second low delay bandpass filter of bandwidth 630 KHz; and(5) an automatic gain control circuit with a dynamic range of 70 dB.

Referring now to FIG. 3, one exemplary embodiment of a cell site system12 includes a first antenna 12-1 that is mounted at an elevatedlocation, preferably on the same structure employed by the cellulartelephone system to mount a cell site antenna. The first antenna 12-1may be independent of the cellular system or may be the antenna employedby the cellular system; i.e., the location system may take a fraction ofthe signal from the cellular system's antenna. A filter/AGC element12-12 could advantageously be located near the antenna 12-1. This wouldreduce cable losses caused by conducting the RF signal over coaxialcable from the antenna to the cell site receiving equipment. The cellsite system 12 further includes an amplifier 12-2 (as discussed above,the amplifier 12-2 may advantageously include sets of filtering and AGCcircuits, one for each control channel); a baseband converter 12-3; asample block 12-4, which includes an upper sideband sampler and a lowersideband sampler; a format block 12-5 (which may be implemented insoftware); a second antenna 12-6, used to receive timing data, forexample, from a global positioning system (GPS); an amplifier 12-7; atiming signal (for example, GPS) receiver 12-8; an automatic gaincontrol (AGC)/control block 12-9; a 5 MHz oscillator 12-10; and acomputer 12-11. The cell site system 12 is coupled to the central site16 (FIG. 2) via a communications line 14.

The cell site system 12 receives one or more cellular telephone signalstransmitted over a control channel from one or more cellular telephones,converts these signals to baseband signals, samples the baseband signals(wherein the sampling frequency is determined by a clock signal providedby AGC/control block 12-9), and formats the sampled signals into framesof data of a prescribed format. The format of the data frames isdescribed below with reference to FIG. 5. The data frames are processedat the central site as described below.

The 5 MHz oscillator 12-10 provides a common reference frequency for allcell site equipment. Its frequency is controlled by the controller 12-9based on measurements made by the controller of the time intervalbetween reception of the one second mark signal from the timing signalreceiver 12-8 and an internally generated one second mark signal.

The computer 12-11 performs three distinct functions concurrently:

(1) It reads the output of square law detectors 54 and 60 inside thebaseband convertor 12-3 (see FIG. 4 and discussion below) and thencalculates the proper control signals to be sent to filter boards 48 and50 (FIG. 4) to adjust the gain and attenuation on these boards with thegoal of maintaining their output power at a constant level.

(2) It receives a signal at each occurrence of a one second mark signalfrom timing signal receiver 12-8. At this time, it reads from controller12-9 the difference in times of arrival of the one second mark signalfrom the timing signal receiver 12-8 and a corresponding one second marksignal internal to controller 12-9. The one second mark signal internalto the controller 12-9 is generated from the 5 MHz oscillator 12-10. Thecomputer then calculates a signal to be sent back to the 5 MHzoscillator to alter its frequency of oscillation with the goal ofcausing the timing receiver's one second mark signal and the internallygenerated one second mark signal to occur simultaneously.

(3) It calculates the information to be encoded in the status bits (seeFIG. 5) and sends that information to the controller 12-9.

Referring to FIG. 4, one preferred embodiment of the baseband convertor12-3 includes an RF input connector 30 to which the elevatedground-based antenna 12-1 (FIG. 3) is connected (via amplifier 12-2 andfilter/AGC 12-12), followed by an attenuator 32 and bandpass filter 34,which sets the level and restricts the frequency response of thebaseband convertor. The filter 34 is followed by a single sideband mixer36 that mixes the RF frequency down to a first IF frequency by action ofa local oscillator signal from buffer amplifier 38. The IF frequency inthe preferred embodiment is in the vicinity of 10 MHz. The output of thebuffer amplifier 38, in addition to being directed to mixer 36, is alsodirected to the divider 42, where it is compared with a 5 MHz referencefrequency from the controller 12-9 (FIG. 3). The output of divider 42 isused to control the frequency of an oscillator 40 so that the collectiveaction of oscillator 40, divider 42, and buffer amplifier 38 provides alocal oscillator signal that is phase locked to the 5 MHz referencefrequency from the controller 12-9. The signal at the first IF frequencyis then directed to single sideband mixer 44, where it is mixed down toa baseband frequency by the action of a computer controlled synthesizer46. Synthesizer 46 is also phase locked to the 5 MHz signal from thecontroller. The upper sideband (USB) output of mixer 44 is then directedto filter/automatic gain control (AGC) amplifier 48, where it isfiltered and its power is continually adjusted to a nominal value. Thelower sideband (LSB) output of mixer 44 is similarly acted upon byfilter/AGC amplifier 50. The output of the filter/AGC amplifier 48includes a 375 KHz signal at 0 dBm directed to the upper sidebandsampler (which is part of sample block 12-4) on wire 52, a separateoutput at -22 dBm directed to a square law detector 54, and a separateoutput directed to a front panel monitor (not shown). The output of thefilter/AGC amplifier 50 includes a 375 KHz signal at 0 dBm directed tothe lower sideband sampler on wire 64, a separate output at -22 dBmdirected to a square law detector 60, and a separate output directed tothe front panel monitor. The baseband converter 12-3 also includes apower distribution board 57 that provides power to the filter/AGCcircuits 48, 50.

FIG. 5 depicts a presently preferred format of the data provided by theformat block 12-5 to the central site 16 (FIG. 2) via the communicationsline 14. As shown, the format block 12-5 provides approximately 1.536Mbps of data to the communications line. Each frame includes 64 syncbits, 48 status bits, 60 kb of sample data (1.5 Mbs divided by 25 framesper second), and approximately 3.6 kb of "filler" data. The 1.5 Mb ofsample data represent the upper sideband and lower side signal samples.The status bits include a time stamp representing the exact time theframe of data was created (which is essentially the same as the time theRF signal was received at the cell site in question).

Central Site System

FIG. 6 is a block diagram of the central site system 16. In onepreferred embodiment, the central site system includes sixteen datainputs each connected to a T1 channel from one of the cell sites. Eachdata input is connected to interface/deformatting circuitry 16-1 (forexample, a T1 CSU) which receives the bipolar T1 signal and outputs databits and a clock signal. The data bits from each channel are clockedinto a FIFO 16-2 by the clock signal from that channel. A computer 16-8selects two of the channel FIFOs through a "select 2 of N" switch 16-3.A sample read clock 16-4 is controlled by the computer 16-8 and a RAMcontrol 16-5 to read sample bits from the previously selected FIFOs. Theoutput of one selected channel FIFO is called "DATA A," and the outputof the other selected channel FIFO is called "DATA B." For the DATA Bsamples, a quadrature channel is calculated by means of an approximateHilbert transform in the quadrature channel generator 16-6, resulting inin-phase output B1 and quadrature phase output B2. A complex correlator16-7 is then used to calculate the correlation coefficient of the DATA Aand DATA B1 signals, and the DATA A and DATA B2 signals, as a functionof the time delay introduced between the DATA A, DATA B1 and DATA A,DATA B2, respectively. The complex correlator may be implemented inhardware or software, or a combination of hardware and software,although hardware is presently preferred because it provides greaterprocessing speed. (One exemplary embodiment of the complex correlator isdescribed below with reference to FIG. 6A.) The computer 16-8 is used toread the resulting correlations periodically. The correlation process,comprising switching the select 2 of N switch, reading the FIFOs,generating quadrature samples, and correlation, is fast enough that asingle complex correlator 16-7 can be used to sequentially process allpairs among the sixteen data input channels.

Because cellular signals are generally weak (e.g., as weak as 6 mW atthe cellular telephone), a reliable and accurate method is required todetect the signal at as many cell sites as possible, and then toaccurately time the same edge of the received signal at each cell site.This ability to accurately time the arrival of the signal is critical tocalculating the delays between pairs of cell sites, and therefore tocalculate position.

Referring now to FIG. 6A, the predetection cross-correlation methodemployed in preferred embodiments of the present invention involvesinputting a sampled strong cellular signal from a first cell site to aninput 72 and inputting a delayed sampled cellular signal from any ofsecond, third, fourth, etc., cell sites to an input 70. The correlatormay be embodied in either hardware or software, as economics dictate fora particular system. The correlator preferably includes sixteen channelsof shift registers 74, two-bit multipliers 76, and counters 78. Multiplecorrelators may be used in series, With each correlator passing bitsthrough its shift register to the next correlator, creating multipledelay channels.

The sampled cellular signal from a second cell site is input to thechained shift registers 74. The outputs from the registers are thenapplied simultaneously to all two-bit multipliers. For each delaychannel, the signal input at 70 delayed by a prescribed number of sampleperiods is applied to each multiplier along with the sampled cellularsignal input at 72. The outputs of the multipliers 76 are input tosummation circuitry, comprising twenty-four-bit counters 78. The outputof each counter is proportional to the strength of the cross-correlationfor a particular relative delay.

By using a plurality of delays, or correlation channels, a large rangeof relative delays can be measured simultaneously. The number of "lags"required is based upon the geographic area to be searched, in terms ofposition determination, the speed of light, and the bandwidth of thereceived signal being applied to the correlator. For example, in theembodiment described above, the control channels are grouped into anupper and lower sideband, each with a bandwidth of 375 KHz. This signalmust be sampled at the minimum Nyquist rate or greater, for example, 750Kbps. If an area of 100 kilometers is to be searched, the number of lagsrequired is ##EQU1##

As discussed above, another embodiment employs individual receivers foreach cellular control channel. If this signal were sampled at 71.428KHz, the number of lags required would be: ##EQU2##

Location System Operation 1. Overview

FIG. 7 is a simplified flowchart of the processing performed by thecentral site system 16. (A detailed flowchart of the signal processingis provided by FIGS. 8A-8E.) First, this system receives a frame of datafrom each of the cell sites. Next, each frame from a given cell site (orthe sampled signal portion of each frame) is cross-correlated with eachcorresponding frame (or the sample portion of each other frame) from theother cell sites. (The term "corresponding" refers to frames beingassociated with the same interval of time). Next, the system generates atable of data identifying the individual signals received by thecellular telephone location system during the interval of timerepresented by the frames of data currently being processed, theindividual signals being represented by the letters "A" , "B" , "C" inFIG. 7. The table further identifies the times of arrival of the signalsat each cell site. These times of arrival are represented by thesubscripts "T1", "T2", "T3". The system therefore identifies the signalsreceived from one or more cellular telephones during a certain intervalof time, and further identifies the time that such signals arrived atthe respective cell sites. This information is then used to calculatetime difference of arrival (TDOA) and frequency difference of arrival(FDOA) data, the latter being employed to estimate velocity. This datais then filtered to remove points the system judges to be erroneous.Next, the filtered TDOA data is employed to calculate the location (forexample, in terms of latitude and longitude) of the individual cellulartelephone responsible for each signal A, B, C. Next, the system decodesthe telephone number corresponding to each cellular telephone whoselocation has been determined. The decoding of the telephone number maybe accomplished with software in computer 16-8 or in hardware (notshown) located at the cell sites. The system employs the strongestsample (highest power) of each signal to determine its telephone number.Thereafter, the location and telephone number data for each telephone iswritten to the database 20 or stored locally via the local disk storagedevice 18 (FIG. 2). Finally, the data may be provided to a user,dispatcher, or billing system. The fields (data) sent to the user,dispatcher, or billing system would preferably include the data bitsrepresenting the dialed digits, the status bits, and the message typefrom the standard cellular control channel message. The data bits couldbe used by the user or a dispatcher to send coded messages to a displayterminal. Thus, in addition to the location services, the locationsystem could provide a limited form of messaging at no incremental cost.

It should be noted that the expression "time difference of arrival," orTDOA, may refer to the time of arrival of a cellular telephone signal atone cell site (for example, cell site A) as determined by a clockreading at that cell site minus the time of arrival of the same cellulartelephone signal at a second cell site (cell site B) as determined by aclock reading at the second cell site. This analysis would be carriedout for all pairs of cell sites A, B. However, the individual times ofarrival need not be measured; only the difference between the signals,times of arrival at the cell sites of a given pair is required. Inaddition, frequency difference of arrival, or FDOA, refers to thefrequency of the cellular signal at a first cell site (cell site A),measured by comparison (effectively) with the cell site's 5 MHzoscillator signal, minus the same quantity for another site (cell siteB). The TDOA data may be used to estimate the latitude and longitude ofthe cellular telephone by calculating that latitude and longitude forwhich the sum of the squares of the difference between the observed TDOAand the TDOA calculated on the basis of the cell site geometry and theassumed cellular telephone location is an absolute minimum, where thesearch of trial latitudes and longitudes extends over the entire servicearea of the system. The FDOA data may be used to measure the velocity(speed and direction of motion) of the cellular telephone. The velocityestimation may be carried out in manner similar to the locationestimation.

2. Control Channel Signal Detection

The inventive method for detecting extremely weak control channelsignals has two preferred embodiments, the selection of which isdependent on the desired capital and operating costs for implementingany particular system. Both methods compensate for the variability of aparticular cellular signal. That is, a transmission on the controlchannel is comprised of multiple fields, such as the cellular telephonenumber, the electronic serial number, any dialed digits, the messagetype, and status and other bits, which make a cellular signal variable.Therefore, the signal cannot be compared against any stored signalbecause each transmission is potentially unique.

In method one, the cell site systems are of higher capital cost, but thecommunication links are of lower speed, for example, 56 Kbps, andtherefore lower operational cost. FIG. 7A schematically depicts thismethod by illustrating the functional components of the cell sitesystems. In this method, cross-correlations are performed at the cellsites in the following manner. For each "strong" signal (e.g., signal"A" ) received on a particular control channel at a particular firstcell site (where "strong" is at least several dB above the noise level),that strong signal is first applied to a signal decoder, such as thatused by the cellular system itself. This decoder demodulates thecellular signal to produce the original digital bit stream which hadbeen modulated to produce the cellular signal. If the decoder cannotdemodulate the digital stream within allowable error thresholds, thisstrong signal is rejected as a starting point for the remaining part ofthis process. This digital bit stream is then modulated by the cell sitesystem to reconstruct the original signal waveform as it was firsttransmitted by the cellular telephone. This reconstructed signalwaveform is cross-correlated against the received signal at the firstcell site. The cross-correlation produces a peak from which an exacttime of arrival can be calculated from a predetermined point on thepeak.

The first cell site system then sends the demodulated digital bit streamand the exact time of arrival to the central site over thecommunications line. The central site then distributes the demodulateddigital bit stream and the exact time of arrival to other cell siteslikely to have also received the cellular transmission. At each of theseother second, third, fourth, etc., cell sites, the digital bit stream ismodulated by the cell site system to reconstruct the original signalwaveform as it was first transmitted by the cellular telephone. Thisreconstructed signal waveform is cross-correlated against the signalreceived at each cell site during the same time interval. In this case,the same time interval refers to a period spanning several hundred toseveral thousand microseconds of time in either direction from the timeof arrival of the strong signal at the first cell site. Thecross-correlation may or may not produce a peak; if a peak is produced,an exact time of arrival can be calculated from a predetermined point onthe peak. This exact time of arrival is then sent via the communicationsline to the central site, from which a delay difference for a particularpair of cell sites can be calculated. This method permits the cell sitesystems to extract time of arrival information from an extremely weaksignal reception, where the weak signal may be above or below the noiselevel. In addition, cross-correlating at cell sites enables the cellsite systems to detect a first leading edge of a cellular telephonesignal and to reject subsequent leading edges caused by multipath. Thevalue of this technique for reducing the effects of multipath will beappreciated by those skilled in the art. This method is appliediteratively to sufficient pairs of cell sites for each strong signalreceived at each cell site for each sample period. For any giventelephone transmission, this method is only applied once. The results ofthe delay pairs for each signal are then directed to the locationcalculation algorithm.

In method two, the cell site systems are of relatively low cost, as theyare primarily responsible for sampling each of the control channels andsending the sampled information back to the central site. However,because no correlation is performed at the cell site, all sampled datamust be sent back to the central site. This requires a high speedcommunications line, for example, a T1 line. The central site receivesdata from all cell sites over identical communications lines, where thedata has been sampled and time stamped using the same time reference(derived from timing receiver). This method is applied iteratively tosufficient pairs of cell sites for each strong signal received at eachcell site for each sample period. This method is only applied once forany given telephone transmission. The results of the delay pairs foreach signal are then directed to the location calculation algorithmdescribed below.

3. Location Calculation

A preferred algorithm used for calculating the location of a cellulartelephone is an iterative process. The first step of the processinvolves creating a grid of theoretical points covering the geographicarea of the cellular telephone system. These points may be, for example,at 1/2 minute increments or some other increment of latitude andlongitude. From each of these theoretical points, the theoretical valuesof delay are calculated for each relevant pair of cell sites. Incalculating the theoretical values of delay, any known site biases areincorporated into the calculation. Known site biases can be caused byany number of mechanical, electrical, or environment factors and mayvary from time to time. The site biases are determined by periodicallylocating the positions of reference cellular transmitters. Since thereference transmitters are, by definition, at known locations, anyvariance in the calculated position of the transmitter from the knownposition is assumed to have been caused by permanent or temporary sitebiases. These site biases are assumed to also affect the measurements ofthe unknown positions of cellular telephones.

Once the theoretical delays are calculated from each theoretical pointon the grid, a least squares difference calculation is performed betweenthe theoretical delays and the actual observed delays for each pair ofcell sites for which delays could be determined by correlation. Theleast squares calculation takes into consideration a quality factor foreach actual delay measurement. The quality factor is an estimatedmeasure of the degree to which multipath or other anomalies may haveaffected that particular delay measurement. (This quality factor isdescribed below.) Therefore, the least squares difference equation takesthe form:

    LSD=[Q.sub.12 (Delay.sub.-- T.sub.12 -Delay.sub.-- O.sub.12).sup.2 +Q.sub.13 (Delay.sub.-- T.sub.13 -Delay.sub.-- O.sub.13).sup.2 +...Q.sub.xy (Delay.sub.-- T.sub.xy -Delay.sub.-- O.sub.xy).sup.2 ]

where, Delay₋₋ T_(xy) is the theoretical between cell sites x and y;Delay₋₋ O_(xy) is the observed delay between cell sites x and y; Q_(xy)is the quality factor the delay measurement cell sites x and y; and LSDis the least squares difference value that is absolutely minimized overthe cellular system's geographic area.

The algorithm searches the entire grid of theoretical points anddetermines the best theoretical point for which the value of LSD isminimized. Starting at this best theoretical latitude-longitude, thealgorithm then performs another linearized-weighted-least-squaresiteration similar to the above-described process to resolve the actuallatitude-longitude to within 0.0001 degrees, or any other chosenresolution. By performing the calculation of latitude-longitude in twosteps, the amount of processing required may be greatly reduced overother approaches.

Those familiar with the art will note that this iterative method ofdetermining position automatically incorporates geometric dilution ofprecision (GDOP) considerations into the calculation of the position ofthe cellular telephone. That is, no separate GDOP table is requiredsince both iterations in the calculation of the grid of theoreticaldelay values also calculate error values.

Cellular telephone signals are subject to multipath and otherimpairments in travelling from the cellular telephone to the variouscell sites. Therefore, the methods described herein incorporatecompensation for multipath. As described above, the symbol rate of thedigital bit stream of the cellular control channel is 10 Kbps, which hasa bit time of 100 microseconds. Published multipath studies have showntypical multipath delays of 5 to 25 microseconds in urban and suburbansettings. The present inventors have discovered that the typical effectof multipath in this case would be to lengthen the bit times of thedigital data streams and that the correlation algorithms described abovecan determine the degree to which a particular transmission has beenimpaired. As mentioned above, when a cross-correlation is performed, aquality factor Q_(xy) may be calculated based upon the size of the peakgenerated by the cross-correlation and the width of the peak, whereQ_(xy) is the quality factor for a particular delay value measurementfor a particular pair of cell sites. This quality factor is useful toweight the least squares calculation used in position determination andthereby mitigate the effects of multipath.

FIGS. 8A-8E are, collectively, a flowchart of the signal processingemployed by the location system to (1) obtain correlation data, (2)obtain time delay and frequency difference data, and (3) calculatelocation data. Referring now to FIG. 8A, which depicts the processingemployed to obtain correlation data, the processing begins by making adetermination whether the received power is above a prescribed thresholdat any cell site. If so, the complex correlator inputs are set toprocess that cell site's data as an autocorrelation, i.e., with bothinputs set to receive the data from the same cell site. The system thenwaits until the correlator is finished computing the autocorrelationdata. Thereafter, the autocorrelation data is Fourier transformed toobtain power spectrum data. Next, the system determines which signalchannels have transmissions and saves the results. Next, a time index iscleared, and then the system sets the correlator input "B" to receivedata from another cell site, leaving the "A" input unchanged. The systemthen waits until the correlator is finished, and then saves thecorrelation results. Thereafter, the system makes a determinationwhether there is a "B" cell site that has not been processed yet. If so,the processing branches back as shown to process the data from that cellsite. If not, the system determines whether power is still beingreceived; if not, this part of the processing is finished; if so, thetime index is incremented and the "B" channel cell site signals areprocessed again, as shown.

The processing performed to obtain time delay and frequency differencedata is depicted in FIG. 8B. The system first sets a first index to asite index for the site at which power was detected. Thereafter, asecond index is set to another site. The time index is then set to afirst time. The correlation data is then stored in a row of a twodimensional array, where the row number corresponds to the time index.Next, the system determines whether another time sample is to beprocessed; if so, the time index is incremented and the system branchesback as shown. If not, the data in the two-dimensional array is Fouriertransformed. The transformed data is then searched for the highestamplitude. An interpolation is then performed to estimate the peak ofthe transformed data. The time delay and frequency difference resultsare then saved. The system then determines whether the second index isto be incremented and, if so, branches back as shown.

FIGS. 8C-8E depict the location estimation process. Referring to FIG.8C, the system first retrieves the observed delays and frequencies. Thecorresponding telephone information is then retrieved. Thereafter, thelatitude and longitude are set to starting latitude, longitude values.Given the starting values, the system then calculates theoretical valuesof delays, taking account of site biases, if any. The system thenobtains the sum of squares of the observed delays minus the computeddelays. This is denoted "X". The system then determines whether this isthe smallest "X" obtained thus far. If not, the system branches forwardas shown to increment the starting longitude value. If this is thesmallest "X", the latitude is saved in "BEST₋₋ LAT" and the longitude issaved in "BEST₋₋ LON". The system then determines whether anotherlongitude and latitude should be tested. If not, the system performs alinearized-weighted-least-squares iteration step, starting at BEST₋₋ LATand BEST₋₋ LON, to determine correction values "LAT₋₋ CORRECTION" and"LON₋₋ CORRECTION".

Referring now to FIG. 8D, the location determination process iscontinued by determining whether the magnitude of LAT₋₋ CORRECTION isless than 0.0001 degrees. Similarly, the system determines whether LON₋₋CORRECTION is less than 0.0001 degree. If either of these tests yields anegative result, the value of LAT₋₋ CORRECTION is added to BEST₋₋ LATand the value of LON CORRECTION is added to BEST₋₋ LON , and theprocessing branches back to perform anotherlinearized-weighted-least-squares iteration step (FIG. 8C). Once themagnitudes of LAT₋₋ CORRECTION and LON₋₋ CORRECTION are less than0.0001, the system proceeds with the velocity calculation by setting aspeed variable to zero and a direction variable to zero (i.e., North).Given these starting values of speed and direction, the systemcalculates theoretical values of frequencies, taking account of any sitebias. The system then computes the sum of the squares of observedfrequencies minus computed frequencies. This sum is denoted "Y". Thesystem then determines whether this value of "Y" is the smallestobtained thus far. If so, the speed is saved in "BEST₋₋ SPEED" and thedirection is saved in "BEST₋₋ DIRECTION". The system then determineswhether another direction should be tested. If so, the direction isincremented and the processing branches back as shown. Similarly, thesystem determines Whether another speed should be tried and, if so,increments the speed and branches back as shown. If the system decidesnot to try another direction or speed, it performs alinearized-weighted-least-squares calculation, starting at BEST₋₋ SPEEDand BEST₋₋ DIRECTION, to determine correction values "SPEED₋₋CORRECTION" and "DIRECTION₋₋ CORRECTION". Thereafter, the systemdetermines whether the magnitude of SPEED₋₋ CORRECTION is less than aspecified value, e.g., one mile per hour. If so, the system determineswhether the magnitude of DIRECTION₋₋ CORRECTION is less than 1°. Ifeither of these tests results in an affirmative answer, the system addsSPEED₋₋ CORRECTION to BEST₋₋ SPEED and adds DIRECTION₋₋ CORRECTION toBEST₋₋ DIRECTION, and the processing branches back as shown to performanother linearized-weighted-least-squares calculation. If SPEED₋₋CORRECTION is less than 1 mile per hour and DIRECTION₋₋ CORRECTION isless than 1°, the system outputs the telephone information, BEST₋₋ LAT ,BEST₋₋ LON , BEST₋₋ SPEED , and BEST₋₋ DIRECTION.

Applications

There are a variety of commercially valuable applications of theinventive technology disclosed herein. For example, in addition to thebasic function of tracking the location of a mobile cellular telephone,the present invention may be employed to offer subscribers billing ratesthat vary on the basis of the location from which a call was made. Asdepicted in FIG. 9, a location tape, containing a record over time ofthe locations of the subscribers' cellular telephones, may be mergedwith a billing tape to produce a modified billing tape. The billing tapecontains data indicating the cost for each telephone call made by thecellular telephones within a certain time period. This cost is basedupon one or more predetermined billing rates. The modified billing datais based upon a different rate for calls made from certain specifiedlocations. For example, the system may apply a lower billing rate fortelephone calls made from a user's home or office.

The invention may also be employed to provide emergency assistance, forexample, in response to a "911" call. In this application, the locationsystem includes means for automatically sending location information toa specified receiving station in response to receiving a "911" signalfrom a cellular telephone.

Further, the invention may be employed in connection with an alarmservice. In this application, a means is provided for comparing thecurrent location of a given telephone with a specified range oflocations and indicating an alarm condition when the current location isnot within the prescribed range.

Yet another application involves detecting a lack of signaltransmissions by a given telephone and in response thereto automaticallypaging the telephone to cause it to initiate a signal transmission. Thisallows the system to locate a telephone that has failed to registeritself with the cellular system. Such a feature could be used, forexample, to generate an alarm for subscribers at remote locations.

Still another application involves estimating a time of arrival of agiven telephone at a specified location. This application is useful, forexample, in connection with a public transportation system to provideestimated times of arrival of busses along established routes. Manyother applications of this feature are also possible.

Conclusion

Finally, the true scope the present invention is not limited to thepresently preferred embodiments disclosed herein. For example, it is notnecessary that all or even any of the "cell site systems" be collocatedwith actual cell sites of an associated cellular telephone system.Moreover, communication links other than T1 links may be employed tocouple the cell site systems to the central site system. In addition,the timing signal receiver need not be a GPS receiver, as other meansfor providing a common timing signal to all cell site systems will beapparent to those skilled in the art. Furthermore, the present inventionmay be employed in connection with many applications not specificallymentioned above. These include stolen vehicle recovery, fleetmanagement, cell system diagnostics, and highway management.Accordingly, except as they may be expressly so limited, the scope ofprotection of the following claims is not intended to be limited to theparticularities described above.

We claim:
 1. A cellular telephone location system for determining thelocations of multiple mobile cellular telephones each initiatingperiodic signal transmission over one of a prescribed set of reversecontrol channels, comprising:(a) at least three cell site systems, eachcell site system comprising: an elevated ground-based antenna; abaseband convertor operatively coupled to said antenna for receivingcellular telephone signals transmitted over a reverse control channel bysaid cellular telephones and providing baseband signals derived from thecellular telephone signals; a timing signal receiver for receiving atiming signal common to all cell sites; and a sampling subsystemoperatively coupled to said timing signal receiver and said basebandconvertor for sampling said baseband signal at a prescribed samplingfrequency and formatting the sample signal into frames of digital data,each frame comprising a prescribed number of data bits and time stampbits, said time stamp bits representing the time at which said cellulartelephone signals were received; and (b) a central site systemoperatively coupled to said cell site systems, comprising: means forprocessing said frames of data from said cell site systems to generate atable identifying individual cellular telephone signals and thedifferences in times of arrival of said cellular telephone signals amongsaid cell site systems; and means for determining, on the basis of saidtimes of arrival differences, the locations of the cellular telephonesresponsible for said cellular telephone signals.
 2. A cellular telephonelocation system as recited in claim 1, wherein said timing signalreceiver comprises a global positioning system (GPS) receiver.
 3. Acellular telephone location system as recited in claim 1, wherein saidcentral site system comprises a correlator for cross-correlating thedata bits of a frame from one cell site system with corresponding databits from each other cell site system.
 4. A cellular telephone locationsystem as recited in claim 3, wherein said central site system furthercomprises:a plurality of data inputs ports each connected to receive asignal from one of said cell site systems; interface/deformattingcircuits for receiving the signals from said input ports and outputtingdata bits and a clock signal; a plurality of FIFO registers each coupledto an interface/deformatting circuit to receive the data bits and clocksignal from that circuit; a switch comprising a plurality of inputports, each input port coupled to an output of one of said FIFOregisters, and a first output port (A) and a second output port (B),said first output port coupled to an input port of said correlator; acomputer operatively coupled to said switch to select two of the inputsto said switch to be output on the output ports of said switch; a RAMcontrol circuit coupled to said computer and said FIFO registers; asample read clock controlled by said computer and said RAM control toread sample bits from previously selected FIFO registers; and aquadrature channel generator comprising an input port coupled to saidsecond output port of said switch and a first output port (B1) and asecond output port (B2), and means for outputting an in-phase signal onsaid first output port (B1) and a quadrature signal on said secondoutput port (B2); wherein said correlator calculates a first correlationcoefficient for said DATA A and DATA B1 signals, and a secondcorrelation coefficient for said DATA A and DATA B2 signals.
 5. Acellular telephone location system as recited in claim 1, wherein saidbaseband convertors each comprise: a first mixer providing anintermediate frequency (IF) signal; a synthesizer providing a localoscillator (LO) signal; a single sideband mixer operatively coupled tosaid first mixer and said synthesizer for converting said IF signal toan upper sideband signal and a lower sideband signal; and means forfiltering said upper sideband and lower sideband signals and providingsaid baseband signals on the basis of the filtered upper and lowersideband signals.
 6. A cellular telephone location system as recited inclaim 1, comprising:first receiver means at a first cell site forreceiving a cellular telephone signal; demodulator means at said firstcell site for demodulating the received cellular telephone signal atsaid first cell site to produce a demodulated digital bit stream; firstmodulator means at said first cell site for modulating the demodulateddigital bit stream to reconstruct the cellular telephone signal as itwas originally transmitted, whereby a first reconstructed cellulartelephone signal is produced; first cross-correlator means at said firstcell site for cross-correlating said reconstructed signal against thecellular telephone signal received at said first cell site to produce afirst peak indicative of a time of arrival of the cellular telephonesignal at the first cell site; means for determining the time of arrivalof the cellular telephone signal at the first cell site on the basis ofsaid first peak and producing first time of arrival data indicativethereof; means for sending the demodulated digital bit stream and firsttime of arrival data from the first cell site to the central site; meansfor distributing the demodulated digital bit stream and first time ofarrival data to a second cell site; second modulator means at saidsecond cell site for modulating the demodulated digital bit stream atthe second cell site to reconstruct the cellular telephone signal as itwas first transmitted by the cellular telephone, whereby a secondreconstructed cellular telephone signal is produced; second receivermeans at said second cell site for receiving said cellular telephonesignal; second cross-correlator means at said second cell site forcross-correlating the second reconstructed signal against the cellulartelephone signal received at the second cell site to produce a secondpeak indicative of a time of arrival of the cellular telephone signal atthe second cell site; means for determining the time of arrival of thecellular telephone signal at the second cell site on the basis of saidsecond peak and producing second time of arrival data indicativethereof; means for sending said second time of arrival data from thesecond cell site to the central site; and means at said central site fordetermining time difference of arrival data on the basis of said firstand second time of arrival data.
 7. A cellular telephone location systemas recited in claim 1, comprising location estimation means for:(1)creating a grid of theoretical points covering a prescribed geographicarea, said theoretical points being spaced at prescribed increments oflatitude and longitude; (2) calculating theoretical values of time delayfor a plurality of pairs of cell sites; (3) calculating a least squaresdifference (LSD) value based on the theoretical time delays and measuredtime delays for a plurality of pairs of cell sites; (4) searching theentire grid of theoretical points and determining the best theoreticallatitude and longitude for which the value of LSD is minimized; and (5)starting at the best theoretical latitude and longitude, performinganother linearized-weighted-least-squares iteration to resolve theactual latitude and longitude to within a prescribed number of degreesor fraction of a degree.
 8. A cellular telephone location system asrecited in claim 7, wherein said calculating step (2) comprisesaccounting for any known site biases caused by mechanical, electrical,or environmental factors, said site biases determined by periodicallycalculating the positions of reference cellular transmitters at knownlocations.
 9. A cellular telephone location system as recited in claim7, wherein said least squares difference is given by:

    LSD=[Q.sub.12 (Delay.sub.-- T.sub.12 -Delay.sub.-- O.sub.12).sup.2 +Q.sub.13 (Delay.sub.-- T.sub.13 -Delay.sub.-- O.sub.13).sup.2 +...Q.sub.xy (Delay.sub.-- T.sub.xy -Delay.sub.-- O.sub.xy).sup.2 ]

where, Delay₋₋ T_(xy) represents the theoretical delay between cellsites x and y, x and y being indices representative of cell sites;Delay₋₋ O_(xy) represents the observed delay between cell sites x and y;Q_(xy) is the quality factor the delay measurement cell sites x and y,said quality factor being an estimated measure of the degree to whichmultipath or other anomalies may have affected a particular delaymeasurement.
 10. A cellular telephone location system as recited inclaim 7, further comprising means for detecting a first leading edge ofa cellular telephone signal and rejecting subsequent leading edges ofsaid cellular telephone signal, whereby the effects of multipath may bereduced.
 11. A cellular telephone location system as recited in claim 1,comprising velocity estimation means for:(1) creating a grid oftheoretical points covering a prescribed range of velocities, saidtheoretical points being spaced at prescribed increments; (2)calculating theoretical values of frequency difference for a pluralityof pairs of cell sites; (3) calculating a least squares difference (LSD)value based on the theoretical frequency differences and measuredfrequency differences for a plurality of pairs of cell sites; (4)searching the entire grid of theoretical points and determining the besttheoretical velocity for which the value of LSD is minimized; and (5)starting at the best theoretical velocity, performing anotherlinearized-weighted-least-squares iteration to resolve the actualvelocity to within a prescribed tolerance.
 12. A cellular telephonelocation system as recited in claim 1, further comprising a database forstoring location data identifying the cellular telephones and theirrespective locations, and means for providing access to said database tosubscribers at remote locations.
 13. A cellular telephone locationsystem as recited in claim 12, further comprising means for providinglocation data to a specific one of said cellular telephones upon requestby the specific telephone.
 14. A cellular telephone location system asrecited in claim 12, further comprising means for merging said locationdata with billing data for said cellular telephones and generatingmodified billing data, wherein said billing data indicates the cost foreach telephone call made by said cellular telephones within a certaintime period, said cost being based upon one or more predeterminedbilling rates, and said modified billing data is based upon a differentrate for calls made from one or more prescribed locations.
 15. Acellular telephone location system as recited in claim 14, wherein thesystem applies a lower billing rate for telephone calls made from auser's home.
 16. A cellular telephone location system as recited inclaim 1, further comprising means for transmitting a signal to aselected cellular telephone to cause said selected telephone to transmita signal over a control channel.
 17. A cellular telephone locationsystem as recited in claim 1, further comprising means for automaticallysending location information to a prescribed receiving station inresponse to receiving a distress signal from a cellular telephone,whereby emergency assistance may be provided to a user in distress. 18.A cellular telephone location system as recited in claim 1, furthercomprising means for comparing the current location of a given telephonewith a prescribed range of locations and indicating an alarm conditionwhen said current location is not within said prescribed range.
 19. Acellular telephone location system as recited in claim 1, furthercomprising means for detecting a lack of signal transmissions by a giventelephone and in response thereto automatically paging said giventelephone to cause said given telephone to initiate a signaltransmission and means for indicating an alarm condition.
 20. A cellulartelephone location system as recited in claim 1, further comprisingmeans for estimating a time of arrival of a given telephone at aprespecified location.
 21. A cellular telephone location system asrecited in claim 1, further comprising means for continuously tracking agiven telephone by receiving voice signals transmitted by said giventelephone over a voice channel and determining the location of saidgiven telephone on the basis of said voice signals.
 22. A ground-basedcellular telephone system serving a plurality of subscribers possessingmobile cellular telephones, comprising:(a) at least three cell sitesequipped to receive signals sent by multiple mobile cellular telephoneseach initiating periodic signal transmissions over one of a prescribedset of reverse control channels; (b) locating means for automaticallydetermining the locations of said cellular telephones by receiving andprocessing signals emitted during said periodic reverse control channeltransmissions; and (c) database means for storing location dataidentifying the cellular telephones and their respective locations, andfor providing access to said database to subscribers at remotelocations.
 23. A ground-based cellular telephone system as recited inclaim 22, further comprising means for providing location data to aspecific one of said cellular telephones upon request by the specifictelephone.
 24. A ground-based cellular telephone system as recited inclaim 22, further comprising means for merging said location data withbilling data for said cellular telephones and generating modifiedbilling data, wherein said billing data indicates the cost for eachtelephone call made by said cellular telephones within a certain timeperiod, said cost being based upon one or more predetermined billingrates, and said modified billing data is based upon a different rate forcalls made from one or more prescribed locations.
 25. A ground-basedcellular telephone system as recited in claim 22, further comprisingmeans for transmitting a signal to a selected cellular telephone tocause said selected telephone to transmit a signal over a controlchannel.
 26. A ground-based cellular telephone system as recited inclaim 22, further comprising means for automatically sending locationinformation to a prescribed receiving station in response to receiving adistress signal from a cellular telephone, whereby emergency assistancemay be provided to a subscriber in distress.
 27. A ground-based cellulartelephone system as recited in claim 22, further comprising means forcomparing the current location of a given telephone with a prescribedrange of locations and indicating an alarm condition when said currentlocation is not within said prescribed range.
 28. A ground-basedcellular telephone system as recited in claim 22, further comprisingmeans for detecting a lack of signal transmissions by a given telephoneand in response thereto automatically paging said given telephone tocause said given telephone to initiate a signal transmission.
 29. Aground-based cellular telephone system as recited in claim 22, furthercomprising means for estimating a time of arrival of a given telephoneat a prespecified location.
 30. A ground-based cellular telephone systemas recited in claim 22, further comprising means for continuouslytracking a given telephone by receiving voice signals transmitted bysaid given telephone over a voice channel and determining the locationof said given telephone on the basis of said voice signals.
 31. A methodfor determining the location(s) of one or more mobile cellulartelephones periodically transmitting signals over one of a prescribedset of reverse control channels, comprising the steps of:(a) receivingsaid reverse control channel signals at at least threegeographically-separated cell sites; (b) processing said signals at eachcell site to produce frames of data, each frame comprising a prescribednumber of data bits and time stamp bits, said time stamp bitsrepresenting the time at which said frames were produced at each cellsite; (c) processing said frames of data to identify individual cellulartelephone signals and the differences in times of arrival of saidcellular telephone signals among said cell sites; and (d) determining,on the basis of said times of arrival differences, the locations of thecellular telephones responsible for said cellular telephone signals. 32.A method as recited in claim 31, further comprising the steps ofstoring, in a database, location data identifying the cellulartelephones and their respective locations, and providing access to saiddatabase to subscribers at remote locations.
 33. A method as recited inclaim 31, further comprising merging said location data with billingdata for said cellular telephones and generating modified billing data,wherein said billing data indicates the cost for each telephone callmade by said cellular telephones within a certain time period, said costbeing based upon one or more predetermined billing rates, and saidmodified billing data is based upon a different rate for calls made fromone or more prescribed locations.
 34. A method as recited in claim 31,further comprising transmitting a signal to a selected cellulartelephone to cause said selected telephone to transmit a signal over acontrol channel.
 35. A method as recited in claim 31, further comprisingautomatically sending location information to a prescribed receivingstation in response to receiving a distress signal from a cellulartelephone, whereby emergency assistance may be provided to a subscriberin distress.
 36. A method as recited in claim 31, further comprisingcomparing the current location of a given telephone with a prescribedrange of locations and indicating an alarm condition when said currentlocation is not within said prescribed range.
 37. A method as recited inclaim 31, further comprising detecting a lack of signal transmissions bya given telephone and in response thereto automatically paging saidgiven telephone to cause said given telephone to initiate a signaltransmission.
 38. A method as recited in claim 31, further comprisingestimating a time of arrival of a given telephone at a prespecifiedlocation.
 39. A method as recited in claim 31, further comprisingcontinuously tracking a given telephone by receiving voice signalstransmitted by said given telephone over a voice channel and determiningthe location of said given telephone on the basis of said voice signals.40. A method as recited in claim 31, comprising the steps of:receiving acellular telephone signal at a first cell site; demodulating thereceived cellular telephone signal at said first cell site to produce ademodulated digital bit stream; modulating the demodulated digital bitstream to reconstruct the cellular telephone signal as it was originallytransmitted, thereby producing a first reconstructed cellular telephonesignal; cross-correlating said reconstructed signal against the cellulartelephone signal received at said first cell site to produce a firstpeak indicative of a time of arrival of the cellular telephone signal atthe first cell site; determining the time of arrival of the cellulartelephone signal at the first cell site on the basis of said first peakand producing first time of arrival data indicative thereof; sending thedemodulated digital bit stream and first time of arrival data from thefirst cell site to a central site; distributing the demodulated digitalbit stream and first time of arrival data to a second cell site;modulating the demodulated digital bit stream at the second cell site toreconstruct the cellular telephone signal as it was first transmitted bythe cellular telephone, thereby producing a second reconstructedcellular telephone signal; receiving said cellular telephone signal atsaid second cell site; cross-correlating the second reconstructed signalagainst the cellular telephone signal received at the second cell siteto produce a second peak indicative of a time of arrival of the cellulartelephone signal at the second cell site; determining the time ofarrival of the cellular telephone signal at the second cell site on thebasis of said second peak and producing second time of arrival dataindicative thereof; sending said second time of arrival data from thesecond cell site to the central site; and determining time difference ofarrival data on the basis of said first and second time of arrival data.41. A method as recited in claim 31, comprising estimating the locationof a cellular telephone by performing the following steps:(1) creating agrid of theoretical points covering a prescribed geographic area, saidtheoretical points being spaced at prescribed increments of latitude andlongitude; (2) calculating theoretical values of time delay for aplurality of pairs of cell sites; (3) calculating a least squaresdifference (LSD) value based on the theoretical time delays and measuredtime delays for a plurality of pairs of cell sites; (4) searching theentire grid of theoretical points and determining the best theoreticallatitude and longitude for which the value of LSD is minimized; and (5)starting at the best theoretical latitude and longitude, performinganother linearized-weighted-least-squares iteration to resolve theactual latitude and longitude to within a prescribed number of degreesor fraction of a degree.
 42. A method as recited in claim 41, whereinsaid calculating step (2) comprises accounting for any known site biasescaused by mechanical, electrical, or environmental factors, said sitebiases determined by periodically calculating the positions of referencecellular transmitters at known locations.
 43. A method as recited inclaim 41, wherein said least squares difference is given by:

    LSD=[Q.sub.12 (Delay.sub.-- T.sub.12 -Delay.sub.-- O.sub.12).sup.2 +Q.sub.13 (Delay.sub.-- T.sub.13 -Delay.sub.-- O.sub.13).sup.2 +...Q.sub.xy (Delay.sub.-- T.sub.xy -Delay.sub.-- O.sub.xy).sup.2 ]

where, Delay₋₋ T_(xy) represents the theoretical delay between cellsites x and y, x and y being indices representative of cell sites;Delay₋₋ O_(xy) is the observed delay between cell sites x and y; Q_(xy)is the quality factor the delay measurement cell sites x and y, saidquality factor being an estimated measure of the degree to whichmultipath or other anomalies may have affected a particular delaymeasurement.
 44. A method as recited in claim 40, further comprisingdetecting a first leading edge of a cellular telephone signal andrejecting subsequent leading edges of said cellular telephone signal.45. A method as recited in claim 31, comprising estimating the velocityof a cellular telephone by performing the following steps:(1) creating agrid of theoretical points covering a prescribed range of velocities,said theoretical points being spaced at prescribed increments; (2)calculating theoretical values of frequency difference for a pluralityof pairs of cell sites; (3) calculating a least squares difference (LSD)value based on the theoretical frequency differences and measuredfrequency differences for a plurality of pairs of cell sites; (4)searching the entire grid of theoretical points and determining the besttheoretical velocity for which the value of LSD is minimized; and (5)starting at the best theoretical velocity, performing anotherlinearized-weighted-least-squares iteration to resolve the actualvelocity to within a prescribed tolerance.